1. Field of the Invention
This invention relates to methods of processing a substrate, and more specifically, to a chemical mechanical scrubbing process using very dilute hydrofluoric acid (HF) for cleaning, for example, semiconductor wafers, including bare silicon wafers where the semiconductor wafers have chemically or thermally grown or deposited oxide layers in which the oxide layers may or may not have been polished by CMP.
2. Background Information
In the manufacture of semiconductor devices, the surface of semiconductor wafers must be cleaned of wafer contaminants. If not removed, wafer contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual.
One problem associated with cleaning silicon wafers whose surface contains a very thin oxide, such as a native (passivation) oxide or a chemically grown oxide from a previous cleaning process, is that the surface metals (e.g., contaminants) to be removed may be on top of the oxide, in the oxide or at the oxide/silicon interface. This native oxide is typically less than 20 .ANG. in thickness. Therefore, in order to remove a certain, predetermined thickness of the thin oxide, a very controlled thin oxide etch must be employed. If the oxide is entirely removed, the surface becomes hydrophobic and may present an adverse surface condition to minimizing surface particulate. Therefore, the goal of the process is to etch as much oxide as possible, removing contamination on the surface of the oxide and incorporated within the native oxide, without removing all of the oxide, thereby maintaining a uniformly hydrophilic surface.
A scrubber that scrubs a wafer on either one or both sides may be used to remove wafer contaminants. The type of cleaning solution (solution) used in the scrubber may depend upon the type of contaminants to be removed, the particular type of wafer to be scrubbed, and/or the particular manufacturer's preferred method for cleaning. For example, some manufacturers require a low level of contamination and may use a chemical solution for scrubbing, while other manufacturers have a higher contamination tolerance (i.e. less contamination need be removed) and may use water for scrubbing.
Double sided scrubbers that use soft sponge like (e.g., PVA) brushes to simultaneously clean both sides of the wafer are widely used for cleaning of silicon wafers, post CMP (chemical mechanical planarization) cleaning and general fab cleaning. The effectiveness of scrubbing wafers with soft brushes to remove surface defects has been demonstrated in both research and production. Ammonium hydroxide solution added to DI Water (DIW) during scrubbing cycle enhances the cleaning performance in many cleaning applications, especially for CMP cleaning. In the case of CMP cleaning, the use of ammonium hydroxide solution helps to remove slurry particles from the wafer surface and prevent brush loading by inducing a negative zeta potential on the particle, PVA brush and the wafer surface, regardless of their zeta potential in DIW. Therefore, the use of dilute ammonium hydroxide solution helps to remove slurry particles, and the metallic contamination associated with them, from the wafer surface. Metallic contamination not associated with slurry particles is present in the liquid part of the polishing slurry or may come from polishing equipment, used polishing pads and pad conditioning tools.
The ammonium hydroxide solution is effective for the removal of surface contamination. However, if contamination resides under the surface, within the oxide bulk, an etching chemistry may be desirable. The etching chemistry would permit removal of a thin oxide layer from the surface and the contaminants incorporated within. This removal of metallic contamination is required for more CMP applications which occur in the front-end of the semiconductor manufacturing process (for example, CMP of polysilicon for polysilicon gates and CMP for shallow trench formation) where removal of contamination, especially metallic contamination, is critical as it may diffuse to the transistor structure and adversely effect the electrical performance of the device.
Other cleaning methods besides the use of scrubbers have been employed. For instance, chemical baths (e.g., wet benches) have been used for cleaning. With chemical baths, cleaning solutions are delivered to the wafer by submersing the wafer within the cleaning solution. Submersing a wafer in the cleaning solution may be disadvantageous because it requires large volumes of chemical solutions which will themselves become contaminated with repeated exposure to "dirty" wafers. Some of the solutions, for example, HF, can be expensive and dangerous to use. Thus, reducing the volume of solution used is desired. In the case of cleaning of silicon and processed semiconductor substrates, for example with HF, accurate control of the etch rate may be required. This is more difficult to accomplish in a bath, containing 50 to 100 wafers, than in a single wafer system with the ability to deliver small amounts of fresh chemicals to each and every wafer.
It may also be necessary for 300 mm processing to engage in a single wafer system to provide more uniform treatment across the surface of the wafer and, therefore, more consistent wafer to wafer process repeatability.